Are the earthquake faults of the San Francisco Bay Area linked miles below
the Bay?

Is stress transmitted from one fault to the next - and if so, how?

Could transmitted stress explain the apparent "pairing" of major quakes in
the region during the 1800s and early 1900s - and does this mean an East Bay
shock is due to follow the San Andreas fault-based earthquake of 1989?

In this most-studied of earthquake regions, surprisingly little is known
about interconnections among the faults visible at the surface, even less
about what those faults look like at depth.

Now, thanks to a massive seismic mapping experiment led by the U.S.
Geological Survey in September, geophysicists have gathered the data that
will make up the first three-dimensional profile of the Bay Area's geologic
layers, 19 miles down to the "Moho" - Mohorovicic discontinuity at the base
of the earth's crust.

Preliminary results from the Bay Area Seismic Imaging Experiment (BASIX)
were presented Dec. 10 at the American Geophysical Union meeting in San
Francisco.

The experiment was "like doing a brain scan of the earth," said geophysics
Prof. Simon Klemperer, who led Stanford's contribution to the collaborative
effort.

Led by Jill McCarthy (Stanford PhD '86) of the U.S. Geological Survey, the
project took advantage of the fact that the navigable waters of San Francisco
Bay overlie parts of the region's major faults. Over a period of two weeks,
the geological survey research vessel S.P. Lee traversed the bay, sending out
sound signals that were bounced off geologic layers beneath the earth. The
signals were picked up by hundreds of land- and water-based seismic
receivers, positioned over 6,000 square miles.

Planned and executed in less than a year, BASIX was a collaboration among
a team of scientists from USGS, Stanford, the University of
California-Berkeley, Lawrence Berkeley Laboratory and Pennsylvania State
University. Co-principal investigators were McCarthy of USGS, Klemperer of
Stanford, Thomas McEvilly of UC- Berkeley and Kevin Furlong of Penn State.

Stanford's contribution was to deploy more than 1000 geophones in 28
arrays specifically aligned to profile strands of major faults. The sensitive
geophones, flown in especially for the experiment, are capable of picking up
sounds as faint as the rustle of wind in leaves or the passing of a curious
snake. Arrayed in groups of six, they can distinguish faint reflections of
acoustic signals from the distant ship. Klemperer, Prof. Emeritus George
Thompson and a crew of graduate students and volunteers tramped the vineyards
of the Napa Valley and the redwoods of Big Basin looking for quiet locations
for the instruments. They mobilized 4 tons of equipment, laid out and
retrieved 10 miles of cable, and drove more than 10,000 miles to collect
their data.

The specially placed land arrays, buoyed underwater microphones and the
region's 200 permanent Calnet seismic stations picked up sound waves from the
S.P. Lee. Differences in the time it took signals to reach a receiver
signaled different layers in the earth's crust: faults show up clearly
because rock layers are different on each side of the fault.

Early results are tantalizing. There is a hint of an offset in San Pablo
Bay that may be a so-far-unsuspected earthquake fault. However, it will take
at least a year to compare and analyze the massive amounts of data collected
by the BASIX instruments and construct a full profile of the Bay Area's
geologic structure.

McCarthy said that among the facts the team hopes to learn are how thick
the earth's crust is in this region, and the composition of its lower layers.

Why isn't this basic information already known? Klemperer explained that
traditional seismic experiments, using dynamite detonated underground, are
expensive and impractical in the crowded, noisy Bay Area. Most current
information about the region's faults comes from seismic recordings during
earthquakes, which only disturb the top 6 to 8 miles of crust. If the BASIX
profile is as detailed as expected, geophysicists may make marine-based
surveys of other regions such as New York's Hudson River or the Chesapeake
Bay.

One major question the scientists expect to be able to answer was posed by
Penn State's Furlong: Did the grinding and overlapping of the earth's
tectonic plates create horizontal links between the region's major faults?

The prevailing assumption is that the San Andreas, Hayward and other major
faults are vertical all the way down to the Moho. Furlong's hypothesis holds
that the region's western faults (the San Andreas and San Gregorio) may turn
a horizontal fault and connect the 6 to 12 mile depth. This horizontal fault
may extend beneath the bay and connect with the eastern faults (the Hayward,
Calaveras and Antioch), finally slanting down through the Moho to the earth's
underlying mantle.

If the faults are only vertical, some stress would be transmitted between
them via the hot lower crust. If they are also horizontally connected, stress
transfer could be much greater. This underground linking of faults could
explain why major Peninsula or East Bay earthquakes in 1836, 1865, 1892 and
1906 were followed within two to six years by major quakes (magnitude 6.5 or
greater) on the opposite side of the bay.

"The beauty of this experiment is its ability to answer simply the
questions that determine earthquake hazard," Klemperer said. He cautioned,
however, that it will not be able to predict with certainty whether strain
might have been transmitted from the San Andreas to the Hayward faults during
the 1989 Loma Prieta earthquake.

The BASIX study is being funded by the National Earthquake Hazards
Reduction Program, USGS, the National Science Foundation and the
participating institutions. Stanford contributed $40,000, using grants from
the university's Office of Technology Licensing, the Dean and Dorothea McGee
Fund and the School of Earth Sciences.

Jennifer Howard is a science writing intern with Stanford News Service

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